NASA-led Study Solves a Methane Puzzle

A reduction in global burned area in the 2000s had an unexpectedly large impact on methane emissions. Find the full animation here. Credit: NASA/GSFC/SVS.› Larger view

NASA-led Study Solves a
Methane Puzzle

A
new NASA-led study has
solved a puzzle involving the recent rise in atmospheric methane, a potent
greenhouse gas, with a new calculation of emissions
from global fires. The new study resolves what looked like irreconcilable differences
in explanations for the increase.

Methane
emissions have been rising sharply since 2006. Different research teams have
produced viable estimates for two known sources of the increase: emissions from
the oil and gas industry, and microbial production in wet tropical environments
like marshes and rice paddies. But when these estimates were added to estimates
of other sources, the sum was considerably more than the observed increase. In
fact, each new estimate was large enough to explain the whole increase by
itself.

Scientist
John Worden of NASA's Jet Propulsion Laboratory in Pasadena, California, and
colleagues focused on fires because they're also changing globally. The area
burned each year decreased about 12 percent between the early 2000s and the
more recent period of 2007 to 2014, according to a new study using observations
by NASA's Moderate Resolution Imaging Spectrometer satellite instrument. The
logical assumption would be that methane emissions from fires have decreased by
about the same percentage. Using satellite measurements of methane and carbon
monoxide, Worden's team found the real decrease in methane emissions was almost
twice as much as that assumption would suggest.

When
the research team subtracted this large decrease from the sum of all emissions,
the methane budget balanced correctly, with room for both fossil fuel and
wetland increases. The research is published in the journal Nature Communications.

Fast Facts:

› Atmospheric methane concentrations are given by their weight in teragrams.

› One teragram equals about 1.1 million U.S. tons -- more than the weight of 200,000 elephants.

› Methane emissions are increasing by about 25 teragrams a year, with total emissions currently around 550 teragrams a year.

Most
methane molecules in the atmosphere don't have identifying features that reveal
their origin. Tracking down their sources is a detective job involving multiple
lines of evidence: measurements of other gases, chemical analyses, isotopic
signatures, observations of land use, and more. "A fun thing about this
study was combining all this different evidence to piece this puzzle
together," Worden said.

Carbon
isotopes in the methane molecules are one clue. Of the three methane sources
examined in the new study, emissions from fires contain the largest percentage
of heavy carbon isotopes, microbial emissions have the smallest, and fossil
fuel emissions are in between. Another clue is ethane, which (like methane) is
a component of natural gas. An increase in atmospheric ethane indicates
increasing fossil fuel sources. Fires emit carbon monoxide as well as methane, and
measurements of that gas are a final clue.

Worden's
team used carbon monoxide and methane data from the Measurements of Pollutants in
the Troposphere instrument on NASA's Terra satellite and the Tropospheric
Emission Spectrometer instrument on NASA's Aura to quantify fire emissions of
methane. The results show these emissions have been decreasing much more rapidly
than expected.

Combining
isotopic evidence from ground surface measurements with the newly calculated
fire emissions, the team showed that about 17 teragrams per year of the
increase is due to fossil fuels, another 12 is from wetlands or rice farming,
while fires are decreasing by about 4 teragrams per year. The three numbers
combine to 25 teragrams a year -- the same as the observed increase.

Worden's coauthors are at the National Center for Atmospheric Research, Boulder,
Colorado; and the Netherlands Institute for Space Research and University of
Utrecht, both in Utrecht, the Netherlands.